Murine leukemia virus (MLV) and lentivirus vectors have been used previously to deliver genes to hematopoietic stem cells (HSCs) in human gene therapy trials. However, these vectors integrate randomly into the host genome, leading to disruption or inactivation of vital host genes. The present invention discloses a novel lentiviral vector system that overcomes this problem by integrating into a host genome in a site-specific manner.